This invention relates to a solid state image sensor unit such as a charge coupled device and, more specifically, to a solid state image sensor unit having horizontal charge transfer portions connected to both ends of a vertical charge transfer portion.
Such a kind of solid state image sensor unit can arbitrarily select a desired one of a plurality of scanning directions of image capture according to a charge transfer pulse serving as a signal for charge transfer. Such a solid state image sensor unit is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 195371/1983.
The conventional solid state image sensor unit comprises a central unit section for mainly performing photoelectric conversion and charge transfer in a unit center area and a peripheral unit section for mainly performing charge detection in a unit peripheral area. The central unit section includes a first semiconductor region and has a vertical charge transfer portion and first and second horizontal charge transfer portions formed at both ends of the vertical charge transfer portion. The peripheral unit section includes a second semiconductor region. The second semiconductor region is connected to a reference potential such as a ground potential. Generally, the first semiconductor region and the second semiconductor region are often called as "channel stopper", respectively.
However, it is found that the conventional solid state image sensor unit has the following problems.
In the conventional solid state image sensor unit, a semiconductor region serving as the main portion of the central unit section is not in physical contact with a second semiconductor region serving as the main portion of the peripheral unit section. However, the first semiconductor region and the second semiconductor region are electrically connected to each other through a well layer.
Here, the electrical resistance of the well layer is considerably higher than the electrical resistances of the first and the second semiconductor regions.
Therefore, the first semiconductor region is connected to the reference potential through the well layer having the high electrical resistance. For this reason, while the solid state image sensor unit is being used, a voltage of the first semiconductor region is easily affected by a charge transfer pulse and then easily varies. In addition, when the voltage of the first semiconductor region varies, charge transfer performance as the maximum amount of charge which can be transferred by the charge transfer portion and/or charge transfer efficiency are reduced because an effective amplitude of the charge transfer pulse becomes small. Therefore, the conventional solid state image sensor unit is narrow in dynamic range and low in resolution.